Thought I’d add my findings in here as well.
I had initial issues getting started with my newly aquired MDX. It got solved as you can see here for reference:
Once I got my device working I stumbled upon some really noticeable high frequency whine noise, but only when using a guitar directly connected to Input 1. The same whine was not happening with Input 2.
At first I thought that something else was also broken because of my initial issue with the bypass relays.
But after a lot of additional troubleshooting I found that the CV/Expression port was set to CV by default. Switching this to Expression pedal (with nothing connected to it physically) removed the whine completely. Interesting, but I guess there was some interference creeping into Input 1 when this was set to CV without anything connected into that socket.
Perhaps this finding can be of help for someone else when troubleshooting noise issues in the MDX.